1. Field of Invention
The present invention relates to dual source refrigeration units. As used in this application "dual source" shall refer to those units which are adapted to alternatively receive electrical power from either an external power source such as a wall receptacle or from a self contained generator. More specifically, the present invention relates to a protective circuit within a combination motor-generator unit which provides for shutdown of the unit as well as malfunction indication under a variety of malfunction situations. As used herein a "combination motor-generator" shall refer to a device which is capable of generating electricity when connected to a prime mover and also capable of acting as a prime mover when connected to an electrical power source.
2. Description of the Prior Art
Refrigeration units used in the trucking industry for cooling trailers or containers must be capable of dual source operation. This type of apparatus usually has an internal power source, such as a gasoline engine, for operation while on the road or away from the shipping dock and it is further adapted to receive electrical power from an external source such as an electrical receptacle while at the shipping dock. Some units have a gasoline engine mechanically connected through a clutch to both a compressor in the refrigeration system and to an electricity generator. Through these connections the engine drives the compressor to power the refrigeration system and simultaneously drives the generator to generate electrical power for the various electrical components of the unit. A separate electric motor has been provided for driving the compressor at dockside when the unit is electrically connected to the power receptacle.
Advanced refrigeration units now use a combination motor-generator rather than having a motor separate from the generator. When the combination unit is not at dockside, operation is similar to the prior unit with the diesel engine powering both the compressor and the motor-generator, the motor-generator then acting as a generator. At dockside the same motor-generator is operated as an electric motor to drive the compressor to eliminate the need for an additional motor. For examples of this type of combination apparatus and the control circuitry utilized therewith see the U.S. Pat. Nos. granted to Martin et al (3,720,842), Harter (3,043,115), Kuklinski (2,907,182) and Shaw (2,881,376).
In a dual source refrigeration unit having a combination motor-generator it was deemed desirable to develop protective circuitry to assure the integrity of the components should a malfunction occur and to simultaneously provide an operational control circuit. During start up of the unit when it is operated in the internal power source mode it is desirable to have the prime mover achieve its operational speed before the compressor of the refrigeration unit is connected to the prime mover, especially since an induction generator will not build up voltage under load. It is further desirable to discontinue unit operation if as a result of improper prime mover speed or other factors the voltage of the current being generated is not within a predetermined range said improper voltage possibly causing overheating of the electric motors in the unit or resulting in a loss of residual magnetism in the core of the motor-generator. Another desirable protective feature is to disconnect the prime mover if the pressure within the refrigeration system is not within a predetermined range. A separate feature includes disconnecting the compressor from the prime mover when the unit is in an auxiliary mode of operation (auxiliary operation includes the unit being operated to defrost the evaporator or to generate heat to raise the temperature of the load). Furthermore, it is desirable to indicate malfunctions to the operator of the unit. None of the references cited above discloses apparatus or a method for providing the appropriate circuitry to obtain these safety and operational features.